The mechanism by which cell-specific gene silencing is established and stably maintained is a major unsolved problem of genetics. Genes relocated to heterochromatin by rearrangement or transposition experience position-dependent mosaic silencing, termed "position-effect variegation" (PEV), which is a model for mitotically stable gene silencing. This project focuses on the heterochromatin-associated protein HP1, which has dosage-dependent effects on PEV. HP1 shares a structural motif (the chromo domain) with other long-range modifiers of transcription, and its phosphorylation is correlated with heterochromatin assembly. The three specific aims of this project are: (1) Determine the role of HP1 phosphorylation in heterochromatic silencing; (2) Determine the functional significance of HP1 chromo domains; (3) Identify proteins that interact with HP1. The phenomenon of genomic imprinting involves allele-specific silencing; as such, heterochromatic silencing provides a model for imprinting. The proposed studies will provide new information which should improve our understanding of the genetics of birth defects and strategies for gene therapies. In yeast and Drosophila, HP1 is required for proper mitotic chromosome segregation. Two human HP1-like proteins have been described, suggesting that a similar HP1-dependent mechanism for mitotic chromosome segregation exists in humans. Thus, a detailed understanding of HP1 structure/function and HP1-associated proteins will likely uncover new information relevant to human chromosome structure and maintenance.